Improve error message when developing `teal` app

### Summary
A descriptive error message is crucial for users when creating a `teal` app or module. Currently, `stopifnot()` is used for assertion in many of teal packages, without overriding the default message. As a result, users will encounter difficulty deciphering the message and may block development progress.

### Definition of Done
* Replace `stopifnot()` with `checkmate`.
* User should receive a consistent and more user-friendly or actionable error message when developing `teal` app.

Triggered by:
insightsengineering/teal.transform/issues/99